The present invention relates to an improved rigid resin powder composition prepared from suspension polymerized poly(vinyl chloride) resin (PVC). This composition which is prepared at substantially elevated temperatures is defined as "densified" in that it has a high bulk density compared to similar PVC powder formulations prepared at moderately elevated temperatures. Further, this invention relates to the method for preparing a densified PVC powder composition which will exhibit optimum processing characteristics in multi-screw extrusion operations.
Resin powder compositions incorporating suspension polymerized PVC have enjoyed widespread usage for many years. As initially developed in the art, these materials, which now are commonly known as "dry blends" or "powder blends," were prepared, for the most part, in blade-type mixers at low shear rates and with the application of heat. More recently, such dry blends have been prepared in high shear mixers wherein the heat necessary for intimate blending of the compounded ingredients is generated within the mixer by friction between impinging resin particles or from contact of the resin particles with metal components in the mixer. By using either the low or high shear mixing method, the maximum temperature attained during the dry blending process usually varies between 180.degree. and 260.degree. F., depending upon the particular formulation prepared.
In U.S. Pat. Nos. 3,567,669 and 3,775,359, issued on Mar. 2, 1971, and Nov. 27, 1973, respectively, there is described and claimed a more efficiently processed rigid PVC powder composition and its preparation. This formulation is prepared in a high shear mixer to a maximum blending temperature of 340.degree.-425.degree. F. Exhibiting a higher bulk density than the earlier dry blends, it may be processed at an accelerated rate in various types of processing equipment to prepare high quality extruded and molded plastic articles. However, when formulated and applied in some processing equipment, e.g., in multi-screw extruders, the compositions prepared according to the aforesaid patent teachings do not consistently give satisfactory results. Oftentimes, these materials tend to bridge in the extruder hopper and exhibit feeding problems, thus exhibiting less than the required free flow characteristics for satisfactory performance in multi-screw extrusion equipment.